Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as energy sources. Among these secondary batteries, a great deal of research associated with lithium secondary batteries having high energy density and discharge voltage is underway, and such a lithium secondary battery is commercially available and widely used.
Depending on the shape of the battery case, secondary batteries are divided into cylindrical batteries and angular batteries in which an electrode assembly is mounted in a cylindrical or angular metal can and pouch-type batteries in which an electrode assembly is mounted in a pouch-type case made of an aluminum laminate sheet. Of these, cylindrical batteries have advantages of relatively high capacity and structural stability.
An electrode assembly mounted in a battery case is a device for generating electricity which is rechargeable and dischargeable, and has a laminate structure of cathode/separator/anode. An electrode assembly is divided into a jelly-roll type electrode assembly fabricated by interposing a separator between a cathode having the shape of a long sheet, to which an active material is applied, and an anode, followed by rolling, and a stack-type electrode assembly fabricated by sequentially laminating a plurality of cathodes with a predetermined size and anodes with a predetermined size such that a separator is interposed between each cathode and each anode. Of these, the jelly-roll type electrode assembly has advantages of being easy to manufacture and having high energy density per weight.
In this regard, FIG. 1 is a cross-sectional perspective view schematically illustrating a general cylindrical battery.
Referring to FIG. 1, a cylindrical secondary battery 100 is fabricated by placing a jelly-roll type electrode assembly 120 in a cylindrical case 130, injecting an electrolyte into the cylindrical case 130 and connecting a top cap 140 provided with an electrode terminal (for example, a cathode terminal, not shown) to the opening top of the case 130.
The electrode assembly 120 has a structure in which a cathode 121, an anode 122 and a separator 123 interposed therebetween are rolled and a cylindrical center pin 150 is inserted into the rolling center thereof (the center of the jelly-roll). The center pin 150 is generally made of a metal material to provide a predetermined strength and has a hollow cylindrical structure in which a sheet material is circularly bent. The center pin 150 fixes and supports the electrode assembly and serves as a passage, allowing emission of gas generated due to an internal reaction upon charging/discharging.
Meanwhile, lithium secondary batteries have a disadvantage of low stability. For example, in the case where a battery is overcharged to about 4.5V or higher, a cathode active material is decomposed, lithium dendrites grow on an anode and an electrolyte is decomposed. These processes involve heat and decomposition reactions and a plurality of side-reactions thus rapidly proceed. Eventually, combustion and explosion of battery may occur.
Accordingly, in order to solve these problems, a general cylindrical secondary battery is provided with a current interruptive device (CID) and a safety vent to interrupt current, when the battery abnormally operates, and reduce an internal pressure in a space provided between an electrode assembly and a top cap.
This mechanism will be described with reference to FIGS. 2 to 4.
Referring to the drawings, the top cap 10 forms a cathode terminal in the form of a protrusion and is provided with a perforated vent. A positive temperature coefficient (PTC) element 20 which greatly increases battery resistance and thereby interrupts current when an internal temperature of the battery increases; a safety vent 30 which protrudes downwardly in a normal state, but protrudes and, at the same time, breaks, resulting in exhaust gas, when the internal pressure of the battery increases; and a connection plate 50, one side of the top of which is connected to the safety vent 30 and the other side of the bottom of which is connected to the cathode of the electrode assembly 40, are arranged under the top cap 10 in this order.
Accordingly, the cathode of the electrode assembly 40 is connected through a lead 42, the connection plate 50, the safety vent 30 and the PTC element 20 in this order to the top cap 10 under normal operation conditions, to apply electricity.
However, when gas is generated from the electrode assembly 40 due to, for example, overcharging, and an internal pressure is increased, as shown in FIG. 3, the shape of the safety vent 30 is inverted and thus protrudes upwardly. At this time, the safety vent 30 is separated from the connection plate 50 and current is thus interrupted. Accordingly, safety is secured in order to prevent further overcharging. Nevertheless, when the internal pressure continues to increase, as shown in FIG. 4, the safety vent 30 breaks, and the pressed gas passes through the broken region and is then exhausted through the vent of the top cap 10, thus preventing explosion of the battery.
Such an operation process depends on the amount of gas generated in the electrode assembly and efficiency of conveyance of the gas to the safety vent. For example, although a great amount of gas is generated, in the case where the gas is not efficiently transported to the safety vent, the desired safety operation process cannot proceed. Furthermore, unless a great deal of gas is generated within a short period of time, and the gas reaches the safety vent and induces a predetermined operation process, the internal pressure of battery rapidly increases, inducing explosion.
Further, generation of gas is induced by decomposition of the electrolyte via heat at high temperature. When heat generated from the batteries rapidly increases within a short period of time, thermal runaway may occur. The thermal runaway phenomenon occurs when a battery is in a continuous electricity application state. As the phenomenon further accelerates, the risk, that the battery may be combusted or exploded, considerably increases, thus disadvantageously causing a serious safety problem.
Accordingly, there is an increasing need for developing a cylindrical secondary battery which can more stably induce an internal gas of battery in a battery abnormal operation state and rapidly exhaust the gas to the outside of battery.